I Have Been Coghing Up Green Mucus And Clearing It

I have been coghing up green mucus and clearing it so I can breath, my doctor said at the moment my lungs sound clear. but I am not having any other viral syptoms, I have a history of double lung pneumonia and some unexplainable craters or corositions they said. thant was 3 months ago, I'm still coughingand nothing is different. what do you think?